A variety of digital magnetometers are known in the art. For example, U.S. Pat. No. 3,396,329 issued Aug. 6, 1968 to Salvi, discloses a magnetometer in which the intensity of weak magnetic fields is a function of frequency differences in sensed signals, but independent of orientation of a vessel in which the magnetometer is installed. U.S. Pat. No. 3,634,946 issued Jan. 18, 1972 to Star, relates to an all digital circuit implementation of a digital compass which operates on the basis of spatial relationships of pulses produced when a sensor is aligned in a reference direction and orthogonal to the Earth's magnetic field. U.S. Pat. No. 4,305,034, issued Dec. 8, 1981 to Long et al., disclosed a magnetometer in which frequency changes are created when a background magnetic field is perturbed by a metal object. This device, however, cannot provide sign information, i.e. whether the field is parallel or antiparallel to the sensor coil. U.S. Pat. No. 4,340,861, issued July 20, 1982 to Bondarevsk et al., discloses that a strong magnetic field will produce frequency differences in an LC circuit.
U.S. Pat. No. 4,851,775, issued Jul. 25, 1989, assigned to Precision Navigation, Incorporated of Mountain View, Calif., incorporated herein by reference, discloses a digital compass and magnetometer having a sensor coil wound on a high magnetic permeability isotropic core. This patent is closely related to the present invention in that the response of the sensor coil to an external magnetic field in conjunction with a resistive element controls the frequency of a relaxation oscillator. In this patent the oscillator driver is a Schmitt-trigger, the output of which provides a signal related to the magnitude of the external magnetic field. This implementation however, suffered from problems with zero offset. The magnetometer zero-offset would drift significantly with temperature and with age.
Another approach that deals specifically with the zero-offset problem is described in European Patent Application A3 0045509 filed Aug. 3, 1981. In that application, the magnetic sensor is an iron core with a winding which determines the frequency of the oscillator. A square wave is laid onto the excitation winding of the magnetic sensor. The detection winding of the magnetic sensor is the inductance of the oscillator which is an LC oscillator. An alternating magnetic bias of the iron core is attained with the square wave voltage. The inductance of the detection winding of the magnetic sensor is dependant upon the magnetic bias. An external magnetic field causes a change in the magnetic bias and thereby a change in the inductance, through which the frequency of the oscillator is raised at one time and lowered at another time. The respective frequencies are ascertained and out of it the difference formed. Thereby, the zero value of the frequency is eliminated, so that the precision of the measurement rises because the variations due to temperature and age of the iron core have been ameliorated. The problem with this approach, however, is that the circuits and their implementation are quite complicated. Furthermore, this system has startup difficulties due to the non-linear inductive sensor.
What is needed is an approach which uses the inherent simplicity of a relaxation oscillator as the principal tool to measure magnetic fields and still achieves the zero offset result, independent of temperature and age.